Deciding what to do next about Iraq is hard - on the merits, and in the politics. It's hard on the merits because whatever comes next, from "surge" to "get out now" and everything in between, will involve suffering, misery, and dishonor. It's just a question of by whom and for how long. On a balance-of-misery basis, my own view changed last year from "we can't afford to leave" to "we can't afford to stay." And the whole issue is hard in its politics because even Democrats too young to remember Vietnam know that future Karl Roves will dog them for decades with accusations of "cut-and-run" and "betraying" troops unless they can get Republicans to stand with them on limiting funding and forcing the policy to change.
By comparison, Iran is easy: on the merits, in the politics. War with Iran would be a catastrophe that would make us look back fondly on the minor inconvenience of being bogged down in Iraq. While the Congress flounders about what, exactly, it can do about Iraq, it can do something useful, while it still matters, in making clear that it will authorize no money and provide no endorsement for military action against Iran.